Tetrix Example

TetrixBoard Class Implementation

In the constructor, we customize the frame style of the widget, ensure that keyboard input will be received by the widget by using Qt::StrongFocus for the focus policy, and initialize the game state:

 TetrixBoard::TetrixBoard(QWidget *parent)
     : QFrame(parent)
 {
     setFrameStyle(QFrame::Panel | QFrame::Sunken);
     setFocusPolicy(Qt::StrongFocus);
     isStarted = false;
     isPaused = false;
     clearBoard();

     nextPiece.setRandomShape();
 }

The first (next) piece is also set up with a random shape.

The setNextPieceLabel() function is used to pass in an externally-constructed label to the board, so that it can be shown alongside the playing area:

 void TetrixBoard::setNextPieceLabel(QLabel *label)
 {
     nextPieceLabel = label;
 }

We provide a reasonable size hint and minimum size hint for the board, based on the size of the space for each block in the playing area:

 QSize TetrixBoard::sizeHint() const
 {
     return QSize(BoardWidth * 15 + frameWidth() * 2,
                  BoardHeight * 15 + frameWidth() * 2);
 }

 QSize TetrixBoard::minimumSizeHint() const
 {
     return QSize(BoardWidth * 5 + frameWidth() * 2,
                  BoardHeight * 5 + frameWidth() * 2);
 }

By using a minimum size hint, we indicate to the layout in the parent widget that the board should not shrink below a minimum size.

A new game is started when the start() slot is called. This resets the game's state, the player's score and level, and the contents of the board:

 void TetrixBoard::start()
 {
     if (isPaused)
         return;

     isStarted = true;
     isWaitingAfterLine = false;
     numLinesRemoved = 0;
     numPiecesDropped = 0;
     score = 0;
     level = 1;
     clearBoard();

     emit linesRemovedChanged(numLinesRemoved);
     emit scoreChanged(score);
     emit levelChanged(level);

     newPiece();
     timer.start(timeoutTime(), this);
 }

We also emit signals to inform other components of these changes before creating a new piece that is ready to be dropped into the playing area. We start the timer that determines how often the piece drops down one row on the board.

The pause() slot is used to temporarily stop the current game by stopping the internal timer:

 void TetrixBoard::pause()
 {
     if (!isStarted)
         return;

     isPaused = !isPaused;
     if (isPaused) {
         timer.stop();
     } else {
         timer.start(timeoutTime(), this);
     }
     update();
 }

We perform checks to ensure that the game can only be paused if it is already running and not already paused.

The paintEvent() function is straightforward to implement. We begin by calling the base class's implementation of paintEvent() before constructing a QPainter for use on the board:

 void TetrixBoard::paintEvent(QPaintEvent *event)
 {
     QFrame::paintEvent(event);

     QPainter painter(this);
     QRect rect = contentsRect();

Since the board is a subclass of QFrame, we obtain a QRect that covers the area inside the frame decoration before drawing our own content.

If the game is paused, we want to hide the existing state of the board and show some text. We achieve this by painting text onto the widget and returning early from the function. The rest of the painting is performed after this point.

The position of the top of the board is found by subtracting the total height of each space on the board from the bottom of the frame's internal rectangle. For each space on the board that is occupied by a piece, we call the drawSquare() function to draw a block at that position.

     int boardTop = rect.bottom() - BoardHeight*squareHeight();

     for (int i = 0; i < BoardHeight; ++i) {
         for (int j = 0; j < BoardWidth; ++j) {
             TetrixShape shape = shapeAt(j, BoardHeight - i - 1);
             if (shape != NoShape)
                 drawSquare(painter, rect.left() + j * squareWidth(),
                            boardTop + i * squareHeight(), shape);
         }
     }

Spaces that are not occupied by blocks are left blank.

Unlike the existing pieces on the board, the current piece is drawn block-by-block at its current position:

     if (curPiece.shape() != NoShape) {
         for (int i = 0; i < 4; ++i) {
             int x = curX + curPiece.x(i);
             int y = curY - curPiece.y(i);
             drawSquare(painter, rect.left() + x * squareWidth(),
                        boardTop + (BoardHeight - y - 1) * squareHeight(),
                        curPiece.shape());
         }
     }
 }

The keyPressEvent() handler is called whenever the player presses a key while the TetrixBoard widget has the keyboard focus.

 void TetrixBoard::keyPressEvent(QKeyEvent *event)
 {
     if (!isStarted || isPaused || curPiece.shape() == NoShape) {
         QFrame::keyPressEvent(event);
         return;
     }

If there is no current game, the game is running but paused, or if there is no current shape to control, we simply pass on the event to the base class.

We check whether the event is about any of the keys that the player uses to control the current piece and, if so, we call the relevant function to handle the input:

     switch (event->key()) {
     case Qt::Key_Left:
         tryMove(curPiece, curX - 1, curY);
         break;
     case Qt::Key_Right:
         tryMove(curPiece, curX + 1, curY);
         break;
     case Qt::Key_Down:
         tryMove(curPiece.rotatedRight(), curX, curY);
         break;
     case Qt::Key_Up:
         tryMove(curPiece.rotatedLeft(), curX, curY);
         break;
     case Qt::Key_Space:
         dropDown();
         break;
     case Qt::Key_D:
         oneLineDown();
         break;
     default:
         QFrame::keyPressEvent(event);
     }

In the case where the player presses a key that we are not interested in, we again pass on the event to the base class's implementation of keyPressEvent().

The timerEvent() handler is called every time the class's QBasicTimer instance times out. We need to check that the event we receive corresponds to our timer. If it does, we can update the board:

 void TetrixBoard::timerEvent(QTimerEvent *event)
 {
     if (event->timerId() == timer.timerId()) {
         if (isWaitingAfterLine) {
             isWaitingAfterLine = false;
             newPiece();
             timer.start(timeoutTime(), this);
         } else {
             oneLineDown();
         }
     } else {
         QFrame::timerEvent(event);
     }
 }

If a row (or line) has just been filled, we create a new piece and reset the timer; otherwise we move the current piece down by one row. We let the base class handle other timer events that we receive.

The clearBoard() function simply fills the board with the TetrixShape::NoShape value:

 void TetrixBoard::clearBoard()
 {
     for (int i = 0; i < BoardHeight * BoardWidth; ++i)
         board[i] = NoShape;
 }

The dropDown() function moves the current piece down as far as possible on the board, either until it is touching the bottom of the playing area or it is stacked on top of another piece:

 void TetrixBoard::dropDown()
 {
     int dropHeight = 0;
     int newY = curY;
     while (newY > 0) {
         if (!tryMove(curPiece, curX, newY - 1))
             break;
         --newY;
         ++dropHeight;
     }
     pieceDropped(dropHeight);
 }

The number of rows the piece has dropped is recorded and passed to the pieceDropped() function so that the player's score can be updated.

The oneLineDown() function is used to move the current piece down by one row (line), either when the user presses the D key or when the piece is scheduled to move:

 void TetrixBoard::oneLineDown()
 {
     if (!tryMove(curPiece, curX, curY - 1))
         pieceDropped(0);
 }

If the piece cannot drop down by one line, we call the pieceDropped() function with zero as the argument to indicate that it cannot fall any further, and that the player should receive no extra points for the fall.

The pieceDropped() function itself is responsible for awarding points to the player for positioning the current piece, checking for full rows on the board and, if no lines have been removed, creating a new piece to replace the current one:

 void TetrixBoard::pieceDropped(int dropHeight)
 {
     for (int i = 0; i < 4; ++i) {
         int x = curX + curPiece.x(i);
         int y = curY - curPiece.y(i);
         shapeAt(x, y) = curPiece.shape();
     }

     ++numPiecesDropped;
     if (numPiecesDropped % 25 == 0) {
         ++level;
         timer.start(timeoutTime(), this);
         emit levelChanged(level);
     }

     score += dropHeight + 7;
     emit scoreChanged(score);
     removeFullLines();

     if (!isWaitingAfterLine)
         newPiece();
 }

We call removeFullLines() each time a piece has been dropped. This scans the board from bottom to top, looking for blank spaces on each row.

 void TetrixBoard::removeFullLines()
 {
     int numFullLines = 0;

     for (int i = BoardHeight - 1; i >= 0; --i) {
         bool lineIsFull = true;

         for (int j = 0; j < BoardWidth; ++j) {
             if (shapeAt(j, i) == NoShape) {
                 lineIsFull = false;
                 break;
             }
         }

         if (lineIsFull) {
             ++numFullLines;
             for (int k = i; k < BoardHeight - 1; ++k) {
                 for (int j = 0; j < BoardWidth; ++j)
                     shapeAt(j, k) = shapeAt(j, k + 1);
             }
             for (int j = 0; j < BoardWidth; ++j)
                 shapeAt(j, BoardHeight - 1) = NoShape;
         }
     }

If a row contains no blank spaces, the rows above it are copied down by one row to compress the stack of pieces, the top row on the board is cleared, and the number of full lines found is incremented.

     if (numFullLines > 0) {
         numLinesRemoved += numFullLines;
         score += 10 * numFullLines;
         emit linesRemovedChanged(numLinesRemoved);
         emit scoreChanged(score);

         timer.start(500, this);
         isWaitingAfterLine = true;
         curPiece.setShape(NoShape);
         update();
     }
 }

If some lines have been removed, the player's score and the total number of lines removed are updated. The linesRemoved() and scoreChanged() signals are emitted to send these new values to other widgets in the window.

Additionally, we set the timer to elapse after half a second, set the isWaitingAfterLine flag to indicate that lines have been removed, unset the piece's shape to ensure that it is not drawn, and update the widget. The next time that the timerEvent() handler is called, a new piece will be created and the game will continue.

The newPiece() function places the next available piece at the top of the board, and creates a new piece with a random shape:

 void TetrixBoard::newPiece()
 {
     curPiece = nextPiece;
     nextPiece.setRandomShape();
     showNextPiece();
     curX = BoardWidth / 2 + 1;
     curY = BoardHeight - 1 + curPiece.minY();

     if (!tryMove(curPiece, curX, curY)) {
         curPiece.setShape(NoShape);
         timer.stop();
         isStarted = false;
     }
 }

We place a new piece in the middle of the board at the top. The game is over if the piece can't move, so we unset its shape to prevent it from being drawn, stop the timer, and unset the isStarted flag.

The showNextPiece() function updates the label that shows the next piece to be dropped:

 void TetrixBoard::showNextPiece()
 {
     if (!nextPieceLabel)
         return;

     int dx = nextPiece.maxX() - nextPiece.minX() + 1;
     int dy = nextPiece.maxY() - nextPiece.minY() + 1;

     QPixmap pixmap(dx * squareWidth(), dy * squareHeight());
     QPainter painter(&pixmap);
     painter.fillRect(pixmap.rect(), nextPieceLabel->palette().background());

     for (int i = 0; i < 4; ++i) {
         int x = nextPiece.x(i) - nextPiece.minX();
         int y = nextPiece.y(i) - nextPiece.minY();
         drawSquare(painter, x * squareWidth(), y * squareHeight(),
                    nextPiece.shape());
     }
     nextPieceLabel->setPixmap(pixmap);
 }

We draw the piece's component blocks onto a pixmap that is then set on the label.

The tryMove() function is used to determine whether a piece can be positioned at the specified coordinates:

 bool TetrixBoard::tryMove(const TetrixPiece &newPiece, int newX, int newY)
 {
     for (int i = 0; i < 4; ++i) {
         int x = newX + newPiece.x(i);
         int y = newY - newPiece.y(i);
         if (x < 0 || x >= BoardWidth || y < 0 || y >= BoardHeight)
             return false;
         if (shapeAt(x, y) != NoShape)
             return false;
     }

We examine the spaces on the board that the piece needs to occupy and, if they are already occupied by other pieces, we return false to indicate that the move has failed.

     curPiece = newPiece;
     curX = newX;
     curY = newY;
     update();
     return true;
 }

If the piece could be placed on the board at the desired location, we update the current piece and its position, update the widget, and return true to indicate success.

The drawSquare() function draws the blocks (normally squares) that make up each piece using different colors for pieces with different shapes:

 void TetrixBoard::drawSquare(QPainter &painter, int x, int y, TetrixShape shape)
 {
     static const QRgb colorTable[8] = {
         0x000000, 0xCC6666, 0x66CC66, 0x6666CC,
         0xCCCC66, 0xCC66CC, 0x66CCCC, 0xDAAA00
     };

     QColor color = colorTable[int(shape)];
     painter.fillRect(x + 1, y + 1, squareWidth() - 2, squareHeight() - 2,
                      color);

     painter.setPen(color.light());
     painter.drawLine(x, y + squareHeight() - 1, x, y);
     painter.drawLine(x, y, x + squareWidth() - 1, y);

     painter.setPen(color.dark());
     painter.drawLine(x + 1, y + squareHeight() - 1,
                      x + squareWidth() - 1, y + squareHeight() - 1);
     painter.drawLine(x + squareWidth() - 1, y + squareHeight() - 1,
                      x + squareWidth() - 1, y + 1);
 }

We obtain the color to use from a look-up table that relates each shape to an RGB value, and use the painter provided to draw the block at the specified coordinates.